1. Field of the Invention
The present invention relates to hold-down springs for holding nuclear reactor internals firmly in place and more particularly to Belleville type spring assemblies for clamping upper and lower reactor internals inside of a reactor vessel while providing a coolant flow path to the reactor vessel head region.
2. Description of the Prior Art
Nuclear reactor cores are usually supported within a cylindrical core barrel arranged within a reactor vessel as a liner and hung from a flange formed where the reactor vessel and reactor vessel head are joined. The core and core barrel are commonly referred to as the lower internals. Coolant flows into the reactor vessel into an inlet annulus and is directed towards the bottom of the core barrel and then up through the core. During operation, the coolant is heated by the core. The heated coolant is then discharged from the reactor vessel as working fluid. Generally, a large pressure differential exists across the core which results in a very large "sailing" or lifting force against the core. This force actually tends to displace the core and its supporting structure. Positioned above the core in the pressure vessel are components known as the upper internals through which the heated coolant may pass before exiting from the pressure vessel. The upper internals are usually contained in a second cylindrical barrel axially aligned above the core barrel. The heated coolant, when passing through the upper internals, exerts a very considerable force against those components as well.
In most pressurized water reactor (PWR) constructions, the upper internals barrel is also supported from the flange formed where the reactor vessel and reactor vessel head are joined. Because of the large size of the structures involved and the significant thermal gradients which exist in the reactor vessel, axial and radial differential expansions occur at the assembly of the vessel and core components. Because of these differential expansions and in large mechanical and hydraulic forces discussed above which act on these structures, the assembly must provide a large enough force to resist displacement.
In addition, it is desirable to maintain the reactor vessel head region at inlet temperatures for safety reasons and to cool the upper internals drive components. Such cooling could only be achieved with a complex system of flow passages with prior designs which utilized a single large Belleville spring to provide a spring load and deflection capability for holding core barrel and upper internals barrel against deflection.
With a large Belleville spring, a clamping load is developed when the reactor vessel head is lowered onto the Belleville spring and drawn down by head studs onto the reactor vessel flange. The spring is typically deflected on the order of only about 0.150 inch resulting in about 460,000 pounds of force to clamp the upper and lower internals against a machined ledge on the inside of the reactor vessel flange. Such loading is sufficient to prevent significant upward motion of the internals during normal operation and during seismic or LOCA events.
However, with large (in the range of 14 to 16 foot diameter) Belleville springs, the loading force is developed over a very short deflection and therefore requires considerable precision. Moreover, large precision machined springs are expensive, difficult to heat treat and, because of their size and shape, difficult to handle, ship and replace. Moreover, with large springs a high stress is developed in the spring over a relatively small deflection which renders its performance vulnerable to stress relaxation after which replacement may be required to maintain adequate loading forces. Replacement is difficult not only because the spring is large but also because it is typically coated with a radioactive crud. Moreover, the size of a single spring is such that the replacement spring must come through a large hatch in the reactor containment resulting in long down times for the reactor.
Moreover, many of the prior large spring designs comprised a 360.degree. structure which required a complex system of flow passages in order to pass inlet coolant to the upper head region. It should be understood that flow rates of up to 16,000 GPM (or on the order of 4% of the inlet flow) have to be accommodated.
Disclosed in U.S. Pat. No. 4,096,034 is a structure for clamping a core barrel and upper internals band against hydraulic displacement by using a few, large Belleville spings mounted in vertical alignment with the wall of the upper internals barrel. No provision is made in the disclosed hold down structure for a Belleville spring assembly which permits an adjustable coolant flow to the upper reactor vessel head region.